Acid–Base Properties of a Freebase Form of a Quadruply Ring-Fused PorphyrinStepwise Protonation Induced by Rigid Ring-Fused Structure

We report herein
unique stepwise protonation at inner imino-nitrogen
atoms of a freebase derivative of a quadruply fused porphyrin (H2QFP), which has been newly synthesized. H2QFP has
been revealed to have the two inner NH protons on the two nonfused
pyrroles by X-ray diffraction analysis and 1H NMR spectroscopy.
The first protonation at one of the two imino-nitrogen atoms of the
fused pyrroles smoothly proceeds with trifluoroacetic acid (TFA) in
CH2Cl2 and the equilibrium constant (K1) of the protonation has been determined to
be (1.3 ± 0.1) × 105 M–1. In
contrast, the second protonation at the other imino-nitrogen atom
is hard to occur unless a large excess amount of TFA is used, as reflected
on a much smaller equilibrium constant, K2 = 7.3 ± 0.3 M–1. The stepwise protonation
is ascribed to the structural rigidity caused by the ring fusion and
the resultant steric repulsion among inner NH atoms of the diprotonated
form. Electrochemical studies have revealed that protonation at the
pyrrole nitrogen atoms caused positive shifts of the reduction potentials
of the QFP derivatives. In addition, the ESR spectrum of the electrochemically
one-electron-reduced monoprotonated QFP derivative showed well-resolved
hyperfine splitting to represent its unsymmetrical electronic structure
due to the monoprotonation.